ARM Technical Support Knowledge Articles

Do I need to connect nTRST to the 20-pin JTAG connector?

Applies to: DSTREAM, RealView ICE and Trace (RVI / RVT)

Answer

It is not strictly necessary to connect nTRST to the 20-pin JTAG connector, but we recommend it as explained in the "System Design Guidelines" chapters of the DSTREAM and RealView ICE user guides.

DSTREAM / RVI units reset the TAP controller by driving nTRST low.  In 'Debug Hardware Config' (formerly RVConfig) you can also select the option "TAP reset via State Transitions" to reset the TAP controller with TCK and TMS.

The difference is that with TCK and TMS only the TAP controller is reset, while nTRST also resets the debug logic in the core, which may result in clearing any existing breakpoints or watchpoints on pre-ARM v7 devices.

Regardless of whether nTRST is taken to a pin or not, it should always be pulsed low at power up to initialize the debug logic.

Article last edited on: 2011-11-04 15:22:00

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